Submerged reverse osmosis system

ABSTRACT

A submerged offshore reverse osmosis desalination apparatus and method uses desalinated product water from the apparatus and an onshore cooler or heat exchanger to provide or improve the cooling of an onshore Rankine Cycle heat engine.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/526,059 filed Nov. 15, 2021, which is a continuation of U.S.application Ser. No. 16/484,323 filed Aug. 7, 2019 (now U.S. Pat. No.11,174,877 B2), which is a national stage filing under 35 U.S.C. § 371of International Application No. PCT/US2018/017599 filed Feb. 9, 2018,which claims priority to U.S. Provisional Application No. 62/457,034filed Feb. 9, 2017, each entitled “SUBMERGED REVERSE OSMOSIS SYSTEM”,the disclosures of each of which are incorporated herein by reference intheir entirety.

TECHNICAL FIELD

This invention relates to water desalination.

BACKGROUND ART

The growth of saltwater (e.g., seawater) desalination has been limitedby the relatively high cost of desalinated water. This high cost is duein part to energy and capital expenses associated with currentdesalination systems. Such systems typically employ an onshore facilitycontaining reverse osmosis (RO) desalination membranes contained inhigh-pressure corrosion-resistant housings and supplied with seawaterfrom a submerged offshore intake system. Very high pressures typicallyare required to drive water through the RO membranes. For example, thewidely-used Dow FILMTEC™ SW30HR-380 reverse osmosis membrane elements(Dow Chemical Co.) require an 800 psi (55 bar) pressure differentialacross the membrane to meet design requirements. In addition to suchhigh pressures, onshore RO units suffer from high power demands,primarily for pressurizing the feedwater to membrane operatingpressures, and for an onshore RO unit typically average about 13.5 kWhper thousand gallons of produced fresh water. The seawater and theconcentrated brine stream produced by a typical onshore RO unit havehigh corrosion potential and consequently require expensive componentsand equipment. The highly-pressurized water flow also increasesmaintenance expenses. Onshore RO units typically also requiresignificant amounts of expensive seaside real estate. Shore-baseddesalination has in addition been criticized for various environmentalimpacts, including entrainment of marine life in the intake water,greenhouse gas production associated with producing the energy required,discharge of a strong brine stream with the potential to harm marinelife, and the use of treatment chemicals that may enter the ocean.

In the 50 years since the invention of semi-permeable RO membranes,various concepts for submerging such membranes and employing naturalhydrostatic water pressure to help desalinate seawater been proposed.Representative examples include the systems shown in U.S. Pat. No.3,456,802 (Cole), U.S. Pat. No. 4,125,463 (Chenowith), U.S. Pat. No.5,229,005 (Fok et al.), (Watkins), U.S. Pat. No. 5,914,041(Chancellor'041), U.S. Pat. No. 5,944,999 (Chancellor'999), U.S. Pat.No. 5,980,751 (Chancellor'751) and U.S. Pat. No. 6,348,148 B1 (Bosley),US Patent Application Publication Nos. 2008/0190849 A1 (Vuong) and2010/0270236 A1 (Scialdone), GB Patent No. 2 068 774 A (Mesple) andInternational Application No WO00/41971 A1 (Gu). An experimental systemis described in Pacenti et al., Submarine seawater reverse osmosisdesalination system, Desalination 126, pp. 213-18 (November, 1999). Itappears however that submerged RO systems (SRO systems) have not beenplaced in widespread use, due in part to factors such as the energy costof pumping the desalinated water to the surface from great depth and thedifficulty of maintaining mechanical moving parts at depth.

From the foregoing, it will be appreciated that what remains needed inthe art is an improved system for RO desalination featuring one or moreof lower energy cost, lower capital cost, lower operating cost orreduced environmental impact. Such systems are disclosed and claimedherein.

SUMMARY OF THE INVENTION

This invention provides in one aspect a submersible reverse osmosisdesalination apparatus comprising (i) one or more osmotic membranes eachhaving an inlet surface and an outlet surface, (ii) a product watercollector in fluid communication with the outlet surface(s), and (iii)an air supply for removing water from the collector via airlift, whereinduring submerged operation of the apparatus the inlet surface(s) aresupplied with saltwater at least partially and preferably solely underhydrostatic pressure, the outlet surface(s) provide desalinated water tothe collector, the collector is in fluid communication with an at leastpartially submerged water discharge conduit, and the air supply liftsdesalinated water from the collector through the conduit in an annularflow regime over a significant portion (e.g., 10% or more) of theairlift depth.

The invention provides in another aspect a method for submerged reverseosmosis desalination, the method comprising supplying air to a submergedreverse osmosis desalination apparatus comprising (i) one or moreosmotic membranes each having an inlet surface supplied with seawater atleast partially under hydrostatic pressure and an outlet surface thatprovides desalinated water, and (ii) a product water collector receivingdesalinated water from the outlet surface(s) and in fluid communicationwith an at least partially submerged water discharge conduit, whereinthe air supply lifts desalinated water from the collector through theconduit in an annular flow regime over a significant portion of theairlift depth.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 a through FIG. 1 e schematically depict various airlift pump flowregimes in a vertical discharge conduit;

FIG. 2 a and FIG. 2 b are graphs illustrating efficiency, capacity andrecommended operating conditions for pumping liquids using an airliftpump;

FIG. 3 depicts various flow regimes overlaid atop a graph of superficialliquid velocity vs. superficial gas velocity for a vertical airlift pumpsystem operated over a range of air and liquid flow rates;

FIG. 4 a through FIG. 4 f schematically depict various airlift pump flowregimes in a horizontal discharge conduit;

FIG. 5 depicts various flow regimes overlaid atop a graph of superficialliquid velocity vs. superficial gas velocity for a horizontal airliftpump system operated over a range of air and liquid flow rates;

FIG. 6 a and FIG. 6 b are respectively schematic cross-sectional viewsof water pumped vertically by an airlift pump operating in a slug flowregime and in an annular flow regime;

FIG. 7 and FIG. 8 are perspective views, partially cut away, of typicalreverse osmosis membrane cartridges;

FIG. 9 and FIG. 10 are schematic sectional views of an SRO desalinationsystem; and

FIG. 11 is a schematic perspective view, partially cut away, of an SROdesalination system.

Like reference symbols in the various figures of the drawing indicatelike elements. The elements in the drawing are not to scale.

DETAILED DESCRIPTION

The recitation of a numerical range using endpoints includes all numberssubsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, 5, etc.).

The terms “a,” “an,” “the,” “at least one,” and “one or more” are usedinterchangeably. Thus, for example, an apparatus that contains “a”filter membrane includes “one or more” such membranes.

The term “air fraction” when used with respect to a two-phase air:liquid(e.g., air:water) flow through a conduit refers to the volumetricfraction, expressed as a percentage, of the air volume over the lengthof the conduit compared to the conduit volume, with the conduit lengthand volume referring to the total length and total volume unlessotherwise specified. Expressed somewhat differently, the air fractionfor such a two-phase flow refers to the air volume as a percent of thetotal volume of air plus liquid in the conduit.

The term “airflow rate” when used with respect to an airlift pumpsupplied by an air compressor refers to the volumetric airflow measuredat the compressor outlet. There are many possible ways to definecompressor operating conditions and specifications (e.g., based onoutlet pressure, flow and temperature). Airflow rates at any given setof conditions and specifications can be converted, using well knownrelationships, to airflow rates at other conditions and specifications.If not otherwise specified herein, airflow rates are measured in unitsof 28.3 Liters (1 cubic foot) per minute at 1 atmosphere (14.73 psi, 1bar or 100,000 Pascals) and 5-10° F. (41-50° C.). The resulting rateswill be numerically somewhat lower than standard cubic feet per minute(sfcm) rates determined at 70° F. (21° C.), but will be used inrecognition of the typical temperatures that will be encountered at theexpected operating depths.

The term “airlift” when used with respect to a pump refers to a deviceor method for pumping a liquid or slurry by injecting air (andpreferably only by injecting air) into the liquid or slurry.

The term “annular flow” when used with respect to a two-phase flowregime in a conduit refers to a regime in which liquid (e.g., water)flows primarily as a film along the conduit wall and gas (e.g., air)flows primarily as a separate phase in the center of the conduit. Thegas phase may contain entrained droplets of liquid, in which case theflow regime may be referred to as “annular flow with droplets” but canstill be regarded as an annular flow regime.

The term “brine” refers to an aqueous solution containing more sodiumchloride than that found in typical saltwater, viz., more than about3.5% sodium chloride.

The term “bubble flow” when used with respect to a two-phase flow regimein a conduit refers to a regime in which gas (e.g., air) primarily flowsas small bubbles within a continuous liquid (e.g., water) phase flowingthrough the conduit. The bubbles may be very small, in which case theflow regime may be referred to as “dispersed bubble flow” or “finelydispersed bubble flow” but may still be regarded as a bubble flowregime.

The term “churn flow” when used with respect to a two-phase flow regimein a conduit refers to a regime between slug flow and annular flow inwhich large bubbles of gas (e.g., air), typically having a diameter nearthe diameter of the conduit and a length ranging up to several times thediameter, flow through the conduit in a chaotic and disordered flowpattern along with liquid that may contain numerous small bubbles.

The term “conduit” refers to a pipe or other hollow structure (e.g., abore, channel, duct, hose, line, opening, passage, riser, tube orwellbore) through which a liquid flows during operation of an apparatusemploying such conduit. A conduit may be but need not be linear, and mayfor example have other shapes including branched, coiled or radiatingoutwardly from a central hub.

The term “depth” when used with respect to an airlift pump (or to acomponent of a submerged apparatus) refers to the vertical distance,viz., to the height of a water column, from the free surface of a bodyof water in which the pump or component is submerged to the point ofpump air introduction or to the location of the component.

The terms “desalinated water” and “fresh water” refer to watercontaining less than 0.5 parts per thousand (ppt) dissolved inorganicsalts by weight. Exemplary such salts include sodium chloride, magnesiumsulfate, potassium nitrate, and sodium bicarbonate.

The terms “efficiency” and “efficiency ratio” when used with respect toan airlift pump intended to pump liquids refer to the ratio of the watermass flow rate to the air mass flow rate. When the context indicates,efficiency may refer to the ratio of output pumping power to therequired input power.

The terms “flow regime” or “flow pattern” when used with respect totwo-phase flow from an airlift pump refer to the type and appearance ofbubbles or other airflow along a specified length of the lift conduit.It will be appreciated that at constant airflow rates the flow regimewill vary within any vertical conduit by depth, with the flow regimebeing less annular (or not annular at all) at the bottom of the conduit,and at sufficiently high airflow ratios becoming more annular or in somecases annular as the depth and the associated hydrostatic pressure inthe conduit at that depth both decrease. It is important to note that atall depths there will be an advantage to the present invention in usingan annular flow regime, due to a reduction in discharge conduitbackpressure and a corresponding reduction in air compressor energydemand.

The term “lift height” when used with respect to an airlift pump refersto the vertical distance from the water surface to the point ofdischarge. For an airlift pump that discharges above the water surface,the lift height will be positive. For an airlift pump that dischargesbelow the water surface, the lift height will be negative.

The term “maximum capacity” when used with respect to an airlift pump ata given submergence ratio refers to the maximum liquid discharge flowrate attainable with a given system configuration using air as theinjection gas.

The term “maximum efficiency” when used with respect to an airlift pumpat a given submergence ratio refers to the efficiency ratio for a givensystem configuration at which increased energy input provides adiminishing increase in water flow rate plotted on the y-axis in atwo-dimensional Cartesian coordinate system (viz., the ordinate) perunit of additional airflow rate plotted on the x-axis (viz., theabscissa). This corresponds to an asymptote for such plot beyond whichthe slope (viz., the ratio of water flow rate to airflowrate)diminishes.

The terms “saltwater” and “seawater” refer to water containing more than0.5 ppt dissolved inorganic salts by weight. In oceans, dissolvedinorganic salts typically are measured based on Total Dissolved Solids(TDS), and typically average about 35,000 parts per million (ppm) TDS,though local conditions may result in higher or lower levels ofsalinity.

The term “slug flow” when used with respect to a two-phase flow regimein a conduit refers to a regime in which gas (e.g., air) primarily flowsthrough the conduit as large bubbles, typically having a diameter at ornear the diameter of the conduit and a length ranging from the diameterto several times the diameter, along with a liquid that may containnumerous small additional bubbles.

The term “submerged” means underwater.

The term “submergence” when used with respect to a submerged airliftpump refers to the vertical distance from the water surface to the (oran) air introduction point.

The term “submergence ratio” when used with respect to an airlift pumprefers to the ratio of submergence to lift height.

The term “submersible” mean suitable for use and primarily used whilesubmerged.

The term “superficial velocity” when used with respect to the flow of afluid in a conduit refers to the volumetric flow rate Q (expressed forexample in m³/s) divided by the conduit cross-sectional area A(expressed for example in m²). When used with respect to a two-phaseflow regime (for example, an air:water flow regime), this definition canbe applied to either phase and calculated to provide a hypothetical flowvelocity for a particular phase as if such phase was the only phaseflowing or present in a given cross-sectional area.

The term “two-phase” when used with respect to flowing substances refersto the simultaneous flow of such substances in two different phases,typically as a gas and a liquid.

The term “water flow rate” when used with respect to an airlift pumpthat pumps water refers to the volumetric airflow at the outlet from thepump discharge conduit.

The term “wide area” when used with respect to dispersal of a fluid(e.g., brine) away from a conduit having a plurality of fluid outlets(e.g., brine outlets) distributed along a length of the conduit, meansdispersal into an area, and typically into a volume, encompassing atleast 5 meters of such length. The disclosed area or volume will alsohave other dimensions (e.g., a width, diameter or height) that willdepend in part upon the direction and velocities of fluid streamspassing through the fluid outlets. Because such other dimensions will beaffected by variable factors including fluid flow rates inside andoutside the conduit, and the overall shape of the dispersed fluid plume,the term “wide area” has been defined by focusing merely on the recitedlength along the recited conduit, as such length typically willrepresent a fixed quantity in a given dispersal system.

Airlift pumping systems may be used for a variety of pumping tasks,including not only the pumping of water but also in undersea miningoperations such as dredging the sea floor to recover gold nuggets ormanganese nodules. “Gas lift” is a term commonly used in oil and gasproduction, including offshore and onshore applications, to raisedesired gaseous or oily products to the surface. Airlift and gas liftsystems can transport solids, e.g., the above-mentioned nuggets andnodules, sand, gold and the like. It is important to note that anannular flow regime cannot be used if solids are to be lifted. This isan important distinguishing feature from many previous air and gas liftsituations where the lifting of solids along with a liquid must beachieved.

Airlift and gas lift systems normally are operated using air or gas andliquid flow rates selected to maximize the amount of desired productobtained per unit of pumping energy expended. For a two-phase systemthat transports air or another gaseous phase and a desired liquidproduct phase, maximum pumping efficiency typically arises when theaverage flow within the conduit carrying the desired liquid product tothe surface represents a so-called “slug” or “churn” flow regime asdiscussed in more detail below. Further details regarding airlift pumpflow regimes may be found for example in Francois et al., A physicallybased model for airlift pumping, Water Resources Research, 32, 8, pp.2383-2399 (1996), Nenes et al., Simulation of Airlift Pumps for DeepWater Wells, Can. J. Chem. Eng., 74, 448-456 (August 1996) and Pougatchet al., Numerical modeling of deep sea air-lift, Ocean Engineering, 35,1173-1182 (2008).

FIG. 1 a through FIG. 1 e schematically depict flow regimes that mayarise in a vertical airlift system at increasing ratios of air mass flowto water mass flow. The discharge water conduit in an actual submergedairlift pump system may include a combination of vertical, horizontal oroblique sections. The nature of the flow regimes that might arise ishowever most easily understood by primarily considering the limitingsituations represented by vertical and horizontal discharge conduits.FIG. 1 a depicts “bubble” flow in a vertical conduit 100 in which airflows as small bubbles 102 dispersed in water 104. Although not shown inthe Drawing, flow regimes that may be referred to as “finely dispersedbubble flow” or “dispersed bubble flow” could also be shown, in whichthe bubbles are generally smaller than bubbles 102. FIG. 1 b depicts“slug” flow in which air flows primarily as large bubbles 106 and to alesser extent as numerous small additional bubbles 108 within water 104.FIG. 1 c depicts “churn” flow in which air flows in a chaotic anddisordered flow pattern primarily as large bubbles such as bubbles 110,112 and 114 and to a lesser extent as numerous small additional bubbles116 within water 104. FIG. 1 d depicts “annular” flow in which air flowsprimarily as a separate phase 118 in the center of conduit 100 and waterflows primarily as a film 120 along the inner wall surface 122 ofconduit 100. FIG. 1 e depicts a further form of annular flow that may bereferred to as “annular flow with droplets”, in which air flowsprimarily as a separate phase 114 containing some entrained droplets 124of water, but the water flows primarily as film 120 along inner wallsurface 122.

FIG. 2 a shows a plot 200 of water flow rate Q_(L) versus air flow rateQ_(G) for a vertical airlift system. At a given lift height, there is aminimum air flow value 202 (designated in FIG. 2 a as “Q_(Gmin)”) thatis required to maintain the initial flow of water at a steady staterate. As the air flow rate Q_(G) and consequently the volume of air inthe discharge water conduit are increased above Q_(Gmin), the flow ofliquid from the discharge water conduit and efficiency both initiallyincrease. At an asymptote represented by point 204, the airlift pumpefficiency, which corresponds to the slope Q_(G)/Q_(G), reaches amaximum value designated as “Q_(Geff), Q_(Leff)”, and thereafterdeclines as the air flow rate increases further. FIG. 2 b illustratesthe pump efficiency η as a function of the energy input from airintroduction, and shows the change in slope for curve 200 withincreasing air flow rate Q_(G). The point of maximum efficiency and thepoint of maximum capacity occur at different air flow rate values. Forpumping liquids, experts in airlift pump design and operation normallyrecommend that the pump be operated between the points of maximumefficiency and maximum capacity. Annular flow is not the most efficientnor does it move the most water. Accordingly, its use as an operatingregime is normally contraindicated.

FIG. 3 depicts the FIG. 1 a through FIG. 1 e flow regimes overlaid atopa graph of superficial liquid velocity vs. superficial gas velocity fora vertical airlift pump system operated over a range of air and liquidflow rates. At small air flow rates and at liquid flow rates up to asuperficial liquid velocity of about 5 m/s, the system operates in theFIG. 1 a bubble flow regime. As the air flow rate increases, the bubblescoalesce to form large bubbles that drive a “slug” of water up theconduit in the FIG. 1 b slug flow regime. Further airflow rate increasescause the large bubbles to become unstable and form the FIG. 1 c churnflow regime. For a vertical conduit pumping liquids, the transition frommaximum efficiency to maximum capacity (see FIG. 2 a ) occurs in thetransition regime between slug flow and churn flow. At yet larger airflow rates, the FIG. 1 d and FIG. 1 e annular flow regimes arise.

FIG. 4 a through FIG. 4 f schematically depict flow regimes that mayarise in a horizontal discharge conduit at increasing airlift ratios ofair mass flow to water mass flow. FIG. 4 a depicts “stratified-smooth”flow in horizontal conduit 400 in which water 104 flows in a smoothstream under air 402. The FIG. 4 a flow regime typically arises only atrelatively low air and liquid velocities. At higher air velocities,waves form in the surface of water 104, providing the “stratified-wavy”flow regime shown in FIG. 4 b . FIG. 4 c depicts a higher air flow ratehorizontal “bubble” flow regime in which air flows as small bubbles 406dispersed in water 104. FIG. 4 d depicts a yet higher air flow rate“elongated bubble” flow regime in which air flows primarily as elongatedbubbles 408 together with smaller bubbles 410 dispersed in water 104.FIG. 4 e depicts a horizontal “slug” flow regime in which air flowsprimarily as large bubbles 412 and to a lesser extent as numerous smalladditional bubbles 414 within water 104. FIG. 4 f depicts horizontal“annular” flow in which air flows primarily as a continuous phase 416 inthe center of conduit 400 and containing some small water droplets 418,with the majority of the water flowing as a film 420 along the innerwall surface 422 of conduit 400.

FIG. 5 depicts the FIG. 4 a through FIG. 4 c , FIG. 4 e and FIG. 4 fflow regimes overlaid atop a graph of superficial liquid velocity vs.superficial gas velocity for a horizontal airlift pump system operatedover a range of air and liquid flow rates. At small air flow rates andat liquid flow rates up to a superficial liquid velocity of about 0.1m/s, the system operates in the FIG. 4 a stratified-smooth flow regime.At somewhat higher air flow rates, the FIG. 4 b stratified-wavy flowregime arises. As the water flow rate increases above that required tomaintain a stratified-smooth flow regime, the FIG. 4 c bubble flowregime eventually arises, with intermittent occurrence of the FIG. 4 delongated bubble flow and FIG. 4 e slug flow regimes, as well asintermittent occurrence of a churn flow regime not shown in FIG. 4 athrough FIG. 4 f . At very high air flow rates and over a relativelywide range of water flow rates, the FIG. 4 f annular flow regime arises.

Air bubbles expand as the depth and hydrostatic pressure decrease. Thusfor a vertical or oblique discharge conduit, the flow regime can varyalong the discharge water conduit length, and may for example representbubble flow at the maximum depth, slug flow or churn flow atintermediate depths and annular flow near the surface. In any event,annular flow is typically characterized as being highly undesirable, forexample by providing “poor pumping efficiency” (see Nenes et al. at p.448) or by rendering the air-lift “impossible” (see Pougatch et al. atp. 1174). Experts consequently do not recommend operating in an annularflow regime when pumping liquids, as doing so is relatively inefficientat best and under some conditions (e.g., when solids such as sand,rocks, or nodules are entrained in the liquid to be pumped) will not bepossible. For example, entrained solids typically will be present inmost oil and gas and all undersea mining applications, and consequentlya slug or churn flow regime typically will be required in suchapplications.

However, when operating an SRO desalination system equipped with anairlift pump to remove desalinated water, this conventional adviceshould be disregarded. FIG. 6 a and FIG. 6 b show in schematiccross-section a conventionally recommended slug flow regime, alongside aconventionally disparaged annular flow regime. An important differencebetween these two flow regimes is that the average weight of thewater:air column in the FIG. 6 a slug flow regime is substantiallygreater than the average weight of the air:water column in the FIG. 6 bannular flow regime, as most of the volume in the latter flow regime isoccupied by air. The reduced average water column weight provided by anannular flow regime significantly decreases backpressure at the bottomof the discharge water conduit. Such reduced backpressurecorrespondingly reduces the required delivery pressure (and for an SROsystem that relies upon hydrostatic pressure, the required depth) neededto accomplish efficient reverse osmosis using conventional reverseosmosis membranes. For example, if conducting SRO usingindustry-standard Dow FILMTEC RO membrane elements and airlift withoutmechanical pump assist to remove product water, a depth of 680 m or moreis preferred in order to provide sufficient hydrostatic pressure forpermeation to take place through the membrane at the recommended 800 psi(55 bar) pressure differential across the membrane. A startup pressureof about 942 psi (65 bar) will be needed to initiate airlift pumping ofthe standing water column in the distribution conduit. However, if theair fraction is raised to about 80%, then an annular flow regime will beobservable along a substantial portion of the discharge water conduitand the required continuous operating pressure (and the backpressure atthe membrane outlet) will drop to about 188 psi (13 bar).

Once continuous desalination and continuous airlift commences, thebackpressure on the downstream side of the osmotic membranes, at thebottom of the delivery conduit, preferably is less than about 580 psi(40 bar), less than about 435 psi (30 bar), less than about 290 psi (20bar) or less than about 218 psi (15 bar).

The pressure, backpressure, air fraction and depth values set out aboveare recommended for use with Dow FILMTEC RO membrane elements. Asdiscussed in more detail below, other RO membranes which have been orare yet to be developed may have different pressure requirements ordifferent performance characteristics, and consequently mightpreferentially be used at other preferred depths or with other airliftvalues to attain SRO with an annular flow regime.

Pumping air at a rate sufficient to achieve an annular flow regimerather than a slug or churn flow regime has several consequences andeffects. An annular flow regime airlift will transport water to thesurface from the SRO system, but at lower pumping efficiency than thepeak potential efficiency that might be attainable using a slug or churnflow regime. However, as discussed above an annular flow regime alsoenables a significant backpressure reduction at the bottom of theairlift. This facilitates deploying and operating the RO membrane at alesser depth than would otherwise be needed when using a slug or churnflow regime and no mechanical pumping. As discussed in more detailbelow, operation at a reduced depth is especially important becauseattaining greater depth typically requires operating further off shore.

The use of an annular flow regime airlift also helps oxygenate and coolthe product water column. Both oxygenation and cooling are assisted bythe large exposed water surface area present during an annular flowregime. The product water may be used for potable water, irrigationwater, process water, water storage, water table replenishment, coolingor heat exchange, and for a variety of other desired uses that will beapparent to persons having ordinary skill in the art. For example,potential cooling or heat exchange applications include providing orimproving the efficiency of air conditioning systems including Sea WaterAir Conditioning (SWAC) systems; providing or improving power plant ordata center cooling; operating or improving the efficiency of OceanThermal Energy Conversion (OTEC) systems; and operating or improving theefficiency of Rankine Cycle heat engines.

The disclosed SRO system may be constructed using a variety ofreadily-available components. FIG. 7 is a perspective view, partiallycut away, of a typical reverse osmosis membrane cartridge. Cartridge 700includes a surrounding pressure-resistant housing 702 encasing innercylindrical housing 706 and spiral-wound membrane 708. Membrane 708typically will have several individual layers arrayed in a sandwich, asdiscussed in more detail below. Saltwater 710 enters cartridge 700through inlet 712, passes into inlet chamber 714, and then passes alongthe length of membrane 708 and laterally along its spiral winding. Inthe course of doing so desalinated water is separated from its salts bymembrane 708 and enters porous central collection tube 716. Concentratedbrine that does not pass through the membrane is collected at outletchamber 718, then removed as brine waste stream 720 via brine outlet722. Desalinated water exits collection tube 716 as stream 724 viaoutlet 726. Typically, an SRO system or other reverse osmosis apparatuswill employ a plurality of such cartridges in series, will collectdesalinated water from each of them in a collector (not shown in FIG. 7) and then will remove desalinated water from the collector by pumpingit through a delivery conduit (also not shown in FIG. 7 ).

FIG. 8 is a perspective view, partially cut away, of another typicalreverse osmosis membrane cartridge. As is the case for the generallysimilar cartridge shown in FIG. 7 , cartridge 800 includes surroundingpressure-resistant housing 802 encasing inner cylindrical housing 806and spiral-wound membrane 808. Saltwater 810 enters cartridge 800through an inlet (not shown in FIG. 8 ) and passes along the length ofmembrane 808 and laterally along its spiral winding. In the course ofdoing so desalinated water is separated from its salts by membrane 808and enters central collection tube 816 through perforations 817.Concentrated brine that does not pass through the membrane is removed asbrine waste stream 820 via brine outlet 822. Desalinated water exitscollection tube 816 as stream 824 via outlet 826. Membrane 808 includesseveral individual layers arrayed in a sandwich. Fabric backing layers830 include a salt-rejecting coating that enables passage of watermolecules through the coating while blocking the passage of salt ions.Grooved plasticized fabric 832 provides spiral channels that conductdesalinated water to perforations 817 and thence into collection tube816. Fabric brine spacer 834 provides a pathway for brine to travel thelength of membrane 808 toward brine outlet 822.

FIG. 9 is a schematic view of one embodiment of the disclosed SROdesalination system. System 900 is submerged in saltwater at anappropriate depth between seafloor 902 and sea surface 904. System 900may if desired rest upon or be anchored to seafloor 902. In anotherembodiment, system 900 may operate at a fixed depth chosen at the timeof installation, or at an adjustable depth that may for example bechanged following startup or changed in response to changing conditions(e.g., changing wave, tidal, thermocline or halocline conditions, orchanges in the operating efficiency of the RO membranes). In a furtherembodiment, system 900 may include a pressure-seeking capability toenable system 900 to increase or decrease its depth in order to obtaindesired hydrostatic pressures, to optimize or adjust RO operatingconditions or to optimize or adjust product water delivery.

System 900 is supplied with compressed air via airline or airlines 906connected to one or more onshore compressors (not shown in FIG. 9 ).Desalinated water product is removed from system 900 via product waterdelivery conduit 908. System 900 includes prefilter 910 for removal ofgross seawater contaminants Filter 910 may contain one or an array ofany suitable filtration devices, for example membranes, nonwoven webs,woven webs, particles, hollow or solid fibers or other filtrationstructures. Where an array of such filtration devices is employed, theymay be configured in series or in parallel or both in series and inparallel. System 900 also includes reverse osmosis unit 912 containingone or an array of reverse osmosis membranes arranged in a preferredparallel configuration for separation of desalinated water and brine. Inother embodiments, the membranes may be configured in series, or both inseries and in parallel. Seawater enters system 900 via inlet screen 1002atop prefilter 910. Airlines 914, 916 and 918 extend from airline(s) 906and may be controlled by on-shore valves, orifice plates or (as shown inFIG. 9 ), by individually actuated valves. Airline 914 supplies lift airto product water delivery conduit 908 for use in directing desalinatedwater product through delivery conduit 908 via airlift pumping. Airline916 supplies purge air to backflush (and if desired, via a furthersuitable valved or otherwise controlled injection point, to flush)prefilter 910. The use of such purge air can remove or prevent thebuildup of contaminants and overcome or avoid clogging, and may becarried out continuously or at any appropriate interval or sequence. Theremoved contaminants may if desired be captured (for example, using asuitable hood or other capture device (not shown in FIG. 9 ), abovescreen 1002. The captured contaminants may for example be used foraquaculture or other purposes. Airline 918 supplies airlift to removeconcentrated brine from reverse osmosis unit 912 via brine removalconduit 920, for dispersal at one or more locations remote from system900, as discussed in more detail below.

FIG. 10 shows system 900 in greater detail. Rough screen 1002 blocks theentry of fish and other large objects into system 900. Coupling 1004joins prefilter 910 to reverse osmosis unit 912, and delivers filteredseawater to reverse osmosis unit 912. Air bubbles 1008 may be suppliedfrom time to time or continuously beneath prefilter 910 to carry out airpurging as discussed above. Fresh product water exits reverse osmosisunit 912 via collector 1006 and enters product water conduit 908,whereupon airlift (supplied from airline 906 via submerged valve 1010and airline 914) can be used to remove the product water using anannular flow regime in a substantial portion of product water conduit908. Manifold 1016 collects brine from reverse osmosis unit 912 anddirects it into brine removal conduit 920. Brine removal conduit 920preferably is provided with airlift using air supplied from airline 906via submerged valve 1014 and airline 918. As discussed above, submergedvalves 1010, 1012 and 1014 can be used to regulate the flow of airthrough airlines 914, 916 and 918 into system 900, and may in theinterest of simplicity and reduced maintenance be eliminated andreplaced by onshore valves or other airflow control measures.

If desired, brine removal conduit 920 may have any other desiredorientation, for example a horizontal or an oblique orientation. Avertical orientation is generally preferred, as such an orientation canreduce the energy required to disperse the brine. Brine removal conduit920 may if desired be combined with or serve as an anchor or tether fora buoy that indicates the SRO system location.

Any desired flow regime may be used in brine removal conduit 920, forexample a slug, churn or annular flow regime. A substantial portion ofbrine removal conduit 920 beyond (viz., above as shown in FIG. 9 andFIG. 10 ) airline 918 preferably includes a plurality of perforations orother openings 1018 in the sidewall of brine removal conduit 920. Theopenings 1018 provide brine outlets through which brine can disperseinto seawater away from brine removal conduit 920. Depending on thesize, shape, extent and axial orientation of such openings and the flowof brine within brine removal conduit 920, seawater may be drawn intosome of the openings 1018 in brine removal conduit 920 and therebyprovide brine dilution within brine removal conduit 920. The disclosedbrine airlift system has several additional advantages discussed in moredetail below, and in Applicant's copending International Application No.WO 2018/148542 A1, filed Feb. 9, 2018 and entitled BRINE DISPERSALSYSTEM, the disclosure of which is incorporated herein by reference.

Although not shown in FIG. 9 and FIG. 10 , persons having ordinary skillin the art will understand that system 900 may include an electricalsupply and appropriate electronic controls to operate air valves orother submerged components, measure desired operating parameters (e.g.,pressures, temperatures, flow rates and the like), and to handle otherelectrically-driven or electrically operated equipment or othersignaling needs. Preferably however the use of submerged electricalcomponents is minimized or eliminated. The disclosed submerged optionalvalves may for example be operated using air pressure provided via oneor more additional air supply lines, or eliminated altogether bysupplying air at appropriately varied pressures from the air compressorsystems, optionally together with appropriate arrangement of therespective depths at which the disclosed airlines inject air into theprefilter, product water stream or brine stream.

FIG. 11 shows a perspective, partially exploded and partially cut-awayview of a prefilter and reverse osmosis assembly 1100 for use in an SROdesalination system. Assembly 1100 includes an array of prefiltrationelements 1102 joined via a coupling 1104 to an array of RO elements1106. The prefiltration elements and RO elements desirably are beprovided without pressure-resistant housings. In the embodiment shown inFIG. 11 , product water flows downwardly through RO elements 1106towards collector 1108, and collector 1108 receives desalinated waterfrom the reverse osmosis elements 1106. In the embodiment shown in FIG.11 , 49 prefiltration elements 1102 and 49 RO elements 1106 are eacharranged in a 7×7 array. Other array sizes, arrangements and membraneflow directions (e.g., upward product water flow through the ROmembranes as disclosed in FIG. 9 and FIG. 10 , or horizontal flow) maybe used as desired to suit the particular application and chosencomponents. Multiple such arrays may be combined in parallel to provideadditional desalination capacity.

The disclosed prefiltration elements can reduce maintenancerequirements. The disclosed SRO system preferably will operate at greatdepth, far from human hands. Some prior proposals for SRO desalinationappear to assume that deep seawater is sufficiently clean to permitdesalination without prefiltration. While it is true that seawater atthe disclosed 680 meter operating depth is typically approximately atleast 90% cleaner than surface water, the remaining 10% may representsufficient contamination to require periodic replacement of the ROmembranes to prevent or overcome clogging or fouling. The disclosedprefilter provides a one-stage and preferably a two-stage prefiltrationtreatment that can prolong RO membrane lifetime. By performingpretreatment at depth rather than offshore, the overall likelihood ofmarine life entrainment is reduced. The prefilter can be periodically orcontinuously back flushed or otherwise purged using air bubbles or anair/water stream to further prolong the prefilter lifetime whilemeanwhile further minimizing or avoiding harm to marine life. Ifdesired, chlorine or ozone may be introduced into the prefilter airliftline, in order to disinfect the prefilter and discourage biofouling.Thus by using the relatively cleaner waters available at the disclosedpreferred depths and a prefilter airline, the disclosed SRO system cansignificantly reduce the prefiltration step(s) and prefilter cleaning orreplacement normally required in shore-based or shallow depth RO unitsand the accompanying capital, operating, real estate and energyrequirements. In some embodiments (e.g., in appropriately clean watersor when using membranes that are less susceptible to fouling),pretreatment may be omitted entirely.

The disclosed SRO system preferably lacks submerged moving parts andespecially wearing parts (e.g., pump impellers, shafts, valves and othercomponents) that might by design or through the failure of a seal orenclosure come into contact with seawater or brine beyond their designedcapability or suitability. In preferred embodiments, the disclosed SROsystem operates entirely without such failure-prone submerged parts aspumps, motors and valves, is composed entirely of seawater-tolerantmaterials, and provides steady-state, continuous or essentiallycontinuous RO desalination using the hydrostatic weight of the oceanabove the membrane or membrane assembly to supply the pressure requiredto drive the pure water through the membrane while leaving most of itssalts behind. Maintenance needs can accordingly be reduced by avoidingfrictional sliding surfaces, pump cavitation, motor or bearing failureand other causes of wear or premature component failure.

The disclosed SRO desalination system will typically be employed with anonshore, offshore (e.g., surface platform-mounted, submerged orship-borne) compressor unit capable of delivering clean compressed airin a quantity required to maintain the above-mentioned annular flowregime. Suitable compressor equipment is available from a variety ofsources that will be familiar to persons having ordinary skill in theart. If desired, these various compressor units may be combined with oneanother or with reserve tanks to provide backup, auxiliary orcomplementary compressed air supply. For example, a submerged or surfaceplatform-mounted compressor unit may be employed in addition to anonshore compressor unit, and any or all of these may if desired bepowered in whole or in part by energy derived from waves, wind orsunlight.

Installation and startup of the disclosed SRO desalination system mayfor example be carried out by attaching an air supply line and thedischarge water conduit to the system and then lowering the system intothe ocean. The discharge water conduit preferably is able to withstandcollapse pressures of at least 970 psi at its deepest reach, and ifdesired its upper reaches need only be configured to withstand thereduced collapse pressures present nearer the surface. The prefilterelements and especially the RO elements preferably are maintained in avertical orientation during system installation and operation. Someprior SRO systems have mounted their RO membranes sideways. If howeverair bubbles are present in the RO membranes during installation orstartup, damage may occur when the membrane becomes pressurized. Avertical orientation helps reduce the likelihood of such damage, as airwill automatically be displaced from the RO membranes during submersionof the SRO apparatus. As the seawater hydrostatic pressure reaches alevel sufficient to begin desalination (e.g., at around 2.76 MPa (400psi), occurring at about 250 meters, for the above-mentioned FILMTECSW30HR-380 RO elements), fresh water will begin to permeate across themembranes into the low-pressure product discharge conduit. At apreferred final operating depth of approximately 680 meters below sealevel, the seawater hydrostatic pressure will be sufficient to providean optimal pressure differential of 5.52 MPa (800 psi) across thedisclosed FILMTEC membrane. At that point the SRO air supply may beturned on. Air will flow to the discharge water conduit airlift, to thebrine airlift, and to the prefilter backflush line. During startup,before annular flow is achieved in the discharge water conduit, thesystem typically will require that air be supplied at higher than normalpressure to enable lifting the filled water column in the dischargewater conduit. Desirably the compressor used during the main runningphase is not called upon to provide such higher pressure directly, andinstead reserve air stored at elevated pressure in one or more separatestorage tanks is employed during the startup phase.

In an alternative embodiment, the discharge water conduit may be closed(for example, by an on-shore or submerged valve) during installation andsubmersion of the SRO apparatus. This will reduce permeation through theosmotic membranes, and will increase pressure within the discharge waterconduit and reduce the extent to which it becomes filled duringsubmersion. The result will be an increased air fraction in thedischarge water conduit column before the initiation of desalination andproduct recovery, and a reduction in the startup air pressure that willbe needed to attain a continuous annular flow regime. Stated somewhatdifferently, the peak air pressure otherwise required to attain acontinuous annular flow regime will be reduced once the conduit isreopened.

Once an annular flow regime is attained (either following a drop in theweight of the corresponding vertical water column in the discharge waterconduit due to the displacement of much of the water in that column byair, or following a reopening of the closed discharge conduit in theabove-mentioned alternative embodiment), the supplied air pressure canbe adjusted (e.g., reduced) to a normal operating range needed forcontinuous pumping of the desalinated water product through the deliveryconduit. In one exemplary embodiment, a pressure of about 68 bar abovethe hydrostatic pressure is employed during the startup phase, and areduced pressure of about 13 bar is employed during the subsequent mainrunning phase when an annular flow regime has been achieved. Thissubstantially lowers the air pressure requirement and consequently theair compressor energy consumption during the main running phase. Higheror lower air pressures may be used during the startup or main runningphase is if desired. Control of the air pressure during startup oroperating conditions may in some embodiments be facilitated by includingan appropriate orifice or orifices in one or more of the prefilter purgeairline(s), product water conduit delivery airline(s) or brine removalconduit airline(s). As discussed in more detail below, it is actuallythe volume of air relative to water and not the pressure that isimportant for attaining the desired annular flow regime operating state.

Meanwhile, as a part of the system design or the startup or operationprocedures, the brine airlift supply should be set, controlled oradjusted. This may be done for example by using an orifice plate in thesystem design, or by using a valve (located at the surface or submerged)for control or adjustments during startup or operation. Either or bothof the brine airlift pressure or air flow volume may be so set,controlled or adjusted, and doing so can provide positive control of theamount of saline (viz., feed) water flowing through the RO membranes tomeet design requirements and conditions. In one embodiment, the brineairlift pressure can be set at the same pressure as the dischargeairlift pressure by injecting air for the brine airlift at anappropriately higher elevation than for the discharge airlift. Thisallows the use of one air pressure line instead of two separate lineshaving different pressures, and helps prevent the accidental injectionof air into the RO membranes.

Importantly, the energy required for SRO airlift is substantially lessthan the energy needed for a mechanical pump to do the same work. SROwith an annular flow regime airlift requires approximately one-tenth thepower needed for surface-based RO using mechanical pumping to providethe primary RO pressure. The same energy comparison is applicable to SROconducted using only hydrostatic pressure to drive the RO membranes andonly mechanical pumping to deliver the product water. The disclosedannular flow regime thus provides a significant reduction in the primaryoperating expense for typical RO saltwater desalination, namely theelectrical power or other energy required to maintain pressure acrossand flow through the RO membranes.

As mentioned above, attainment of the desired annular flow regime doesnot depend upon controlling air pressures, but rather on controlling theratio of air to water. For a conventional airlift pump lifting liquid ina slug or churn flow regime, the air fraction typically will be about40%. During continuous operation of the disclosed SRO apparatus, an airfraction is selected to provide an annular flow regime over asignificant portion of the airlift, for example an air fraction of atleast 60%, typically about 80% and in some embodiments as high as 90%,95% or even 99%. The disclosed SRO desalination system thus employs anairlift pump operated at substantially higher air/water volumetricratios (and thus, at higher air fractions) than those employed to attaina slug or churn flow regime. For example, the disclosed SRO apparatusmay be operated using air:water volumetric ratios (as averaged along thelength of the product water delivery conduit) of at least about 60:40,at least about 70:30, at least about 80:20, at least about 85:15, or atleast about 90:10. The apparatus may be operated using air:watervolumetric ratios as high as about 99:1, as high as about 95:5, or ashigh as about 91:9. The use of such air:water ratios facilitatesformation of an annular flow regime (including in some embodiments flowregimes that may be identified as “wispy annular flow” or “annular withdroplets”) over at least a substantial length of the discharge waterconduit, for example over at least the upper 20%, upper 30%, upper 40%,upper 50% or upper 60% of the discharge water conduit, and in someembodiments over at least the upper 90%, upper 80% or upper 70% of thedischarge water conduit. This reduces the average density and overallweight of material in the discharge water conduit and consequentlyreduces backpressure at the bottom of the conduit on the low pressureside of the reverse osmosis membrane. This is especially helpful for anSRO system that relies partially or preferably wholly upon hydrostaticpressure to drive seawater through its RO membranes. Extra requireddepth is undesirable because in addition to having to submerge the SROsystem to a greater depth, it may also be necessary to place the SROsystem further from shore and consequently to incur additional expensesto lengthen the air supply line(s) and product delivery conduit. Also,if sufficient depth is not available at the chosen location, thenmechanical pumping assist may be needed to attain sufficientdifferential pressure across the RO membranes. In other embodiments,mechanical pumping assist may be employed along with airlift in a hybridsystem designed to operate at lesser depths or shorter offshoredistances than those required for SRO operation using only hydrostaticpressure.

The disclosed airlift will provide surplus air at the top of the productwater distribution column Owing to the cooling effects of seawatersurrounding the airline and associated product water delivery conduit,and the temperature reduction that accompanies volumetric expansion asthe airlift air travels toward the surface, the surplus air represents auseful supply of chilled air that can be used for the above-mentionedcooling or heat exchange applications. The surplus air may also bereused by sending it to a compressor to provide further airlift or toreduce the required operating depth for the disclosed SRO system.

If desired, the disclosed airlift may be assisted during startup orduring steady-state operation by applying a vacuum to the productdelivery conduit, e.g., at or near the top of the product deliveryconduit. The withdrawn air may if desired be reused for airlift or forother purposes.

Conventional onshore RO units often use treatment chemicals toneutralize the product water to an acceptable pH (for example, to a pHof about 6 to about 8). The use of airlift and an annular flow regime inthe present system exposes the product water to ample amounts of air.Doing so can neutralize the product water without requiring the use ofpH neutralization chemicals, thus saving money and reducing theintroduction of chemicals into the product water. On the other hand, thedisclosed airlift may if desired be used to provide alternative oradditional methods for water treatment. For example, chlorine may beintroduced into the airlift line supplying the product water deliveryconduit, in order to chlorinate and disinfect the product water anddiscourage downstream biofouling.

For use with Dow FILMTEC and several other commercially-available ROmembranes, the disclosed SRO desalination system preferably is operatedat a depth of at about 680 m. Doing so will create a high-pressure(approximately 68-bar) condition on the high-pressure side of thesemi-permeable RO membrane. When used with a presently preferredbackpressure of 13 bar or less on the discharge side of the membrane,this will result in a pressure differential across the membrane of 55bar (approximately 800 psi) or more. In situations of higher- orlower-salinity waters, these depth and pressure values may vary. Theinlet pressure will in any event normally be the ocean hydrostaticpressure at the chosen SRO operating depth.

The preferred depth and pressure values set out above may vary insystems that take advantage of future membrane developments enabling orrequiring lower or higher differential pressures or higher or lowermembrane backpressures. Adjustments to accommodate such developments mayincrease or decrease the preferred operating depth for the disclosed SROsystem. For many membranes, the pressure on the low-pressure sidetypically will not change appreciably with depth, and consequentlychanging the depth of operation may suffice to adjust the differentialpressure across the membrane and achieve optimal operating conditions.In a preferred embodiment, desalination in the disclosed SRO system isdriven entirely by hydrostatic seawater pressure on the high-pressureside of the RO membranes, and a low-pressure condition is maintained onthe outlet or product side of the membranes by a flow of compressed airsupplied from the surface, at a flow rate and pressure sufficient tocreate an annular flow regime for air and water over a significantportion of the delivery conduit and adequately evacuate the desalinatedwater product. Once such a system is at the proper depth and air isflowing at the correct volume and pressure, the system preferablycontinuously desalinates seawater and delivers pure water to the surfacewith no moving parts below the waterline that would be subject to wearor breakage.

The disclosed SRO system may if desired be operated at depths less than680 m. If doing so using RO membranes whose pressure and pressuredifferential requirements are like those of the above-mentioned DowFILMTEC membrane elements, then it may be necessary to provide asuitable pressure assist on the inlet side of the RO membranes (or asuitable vacuum assist on the outlet side) in order to achieve efficientdesalination. Such an assist may be accomplished using supplied airpressure, the above-mentioned vacuum, or if need be by using a submergedmechanical pump. Operating depths may if desired be increased beyond thedepth required for pump-free desalination (e.g., beyond 680 m), with acorresponding decrease in the required air fraction to achievesufficient operating differential pressure. Exemplary depths foroperation of the disclosed SRO desalination system are for example fromjust below the surface (e.g., from about 10 m), from about 100 m, fromabout 300 m, or from about 500 m, and up to about 2,000 m, up to about1,500 m or up to about 1,000 m. Preferred depths are from just below thesurface to about 1500 m depth. Near the surface, the hydrostaticpressure of the ocean will need to be augmented by mechanical pumping toprovide the trans-membrane pressure differential needed for reverseosmosis. The marginal (viz., incremental) energy benefits of increaseddepth are greatest near the surface. As depth increases, the energybenefits accruing from mechanical pumping rapidly decrease, making thisan unattractive approach in view of the accompanying increasedcomplexity of an RO system employing both hydrostatic pressure andmechanical pumping. This has been however an approach taken in someprevious SRO designs.

In one preferred embodiment, the disclosed SRO apparatus is deployed inan ocean trench or dropoff (for example, the Monterey Submarine Canyon,Puerto Rico Trench, Ryukyu Trench and other accessible deep sea sitesthat will be familiar to persons having ordinary skill in the art), neara populated area in need of desalinated water. The SRO inlet surfacesneed not be placed at depth of the trench floor, and may instead bepositioned along the trench wall at a depth sufficient to enable the useof hydrostatic pressure to drive seawater through the osmotic membranes.

An optimal operating depth may be determined based on airlift airfraction and membrane operating characteristics. When using thedisclosed Dow FILMTEC membrane elements, a 680 m depth and an 80% airfraction represent preferred choices. Greater depths will permit the useof smaller air fraction values (for example, at 1500m the required airfraction to attain an annular flow regime is about 20%), but will alsolessen the number of available deep water locations at which thedisclosed SRO system may be deployed.

Operation at appropriate depths can greatly reduce or eliminate thelikelihood of algal bloom contamination, which can cause conventionalshore-based plants with shallow water intakes to shut down in order toavoid toxins and clogging. Operation at such depths can also minimize oreliminate the loss of marine life, as most marine organisms are foundwithin the photic zone (depending upon water clarity, corresponding todepths up to about 200 m) and thus at deeper depths will not be drawninto the SRO system intake or against an intake screen.

The cold feedwater (e.g., 5-10° C. water) typically encountered at theabove-mentioned recommended SRO operating depths can provide severaluseful advantages. For example, the feedwater is relatively free fromcritical organic and inorganic contaminants. It carries almost noorganic matter or chlorophyll and thus contains virtually no bacteria,while still retaining valuable nutrients from the ionic minerals andtrace elements present at the disclosed pressures and depths. A furtheradvantage arises in connection with boron removal, which is importantfor irrigation water and health purposes. Boron is present in seawater,and at conventional RO operating temperatures such as are used inonshore RO units, enough boron may pass through the RO membrane toinhibit the growth of plants. Boron removal to agricultural standards of0.5 mg/liter in a conventional RO facility may require double treatmentof the water using a second RO pass, thus increasing capital andoperating costs. Boron removal by reverse osmosis is however highlytemperature-dependent, with lower amounts of boron and its salts passingthrough the membranes at colder temperatures. For example, boratepassage may be reduced by several percentage points for every reductionof 10° C. in feedwater temperature. Placement of the disclosed SROdevice in cold deep water consequently may help produce higher-qualitydesalinated water by improving the removal of boron and its salts whilesaving the energy, capital, and maintenance costs required for a doubletreatment system. Cold feedwater can also result in less overall saltpassage through the membrane, allowing for remineralization of theproduct water for taste reasons while maintaining a low level of TDS tomeet regulatory requirements. In addition, the use of cold feedwater cannearly eliminate the scaling of membranes by mineral deposition, asmeasured by the Langelier Index. Membrane scaling can be a problem withshore-based, shallow-intake RO units, and reduces system efficiency andlifetime. In the disclosed SRO system, scaling is minimized because CO₂will tend to be in equilibrium at the 5-10° C. temperatures at which theRO membranes may be operating. This can eliminate the need for theanti-scaling chemicals that often are employed in shore-based RO units.Biofilm growth, another form of membrane fouling, is alsotemperature-dependent, with more biofilm forming at warmer temperatures,and less at the low-temperature operating environment of the disclosedSRO system. Biological activity and hence biological fouling are thusreduced due to the use of water from a region having no light, lowoxygen, and cold water temperatures.

In some prior SRO designs, especially those that rely on a pressure pumpto force seawater through the membranes, thick pressure-resistantvessels are employed to contain the high pressures needed for membraneseparation. In preferred embodiments of the present SRO desalinationsystem, the prefiltration elements and RO membranes will not requirepressure-resistant vessels, as they will already be immersed at asufficiently high pressure in the fluid to be purified. Desirably thedisclosed SRO system merely maintains a sufficiently low pressure on themembrane discharge side, and a sufficient inlet side-outlet sidepressure differential, so as to allow proper membrane operation withoutthe use of a surrounding pressure-resistant vessel.

The disclosed SRO system can produce significantly lower concentrationsof salt in the brine stream than will be the case for conventional RO,as the elimination of the requirement for pressure vessels permits theRO membranes to be arrayed in parallel rather than the typical seawaterdesalination industry practice of 5-7 membranes in a serial arrangement.A parallel array eliminates a common failure point in conventional ROsystems, namely the o-ring interconnections between membranes. Aparallel arrangement also permits higher product water production permembrane. In addition, a parallel membrane arrangement creates much lesssalty brine than a train of single membranes operating in series, andthis salinity can be adjusted by adjusting the brine airlift operatingparameters. The disclosed SRO system's ability to achieve low brinesalinity would be beneficial to sea life and would allow easier brinedilution. For example, using seawater containing 35,000 ppm TDS, thedisclosed system may provide brine containing 38,043 ppm TDS (a 9%increase) versus the near-doubling in discharge stream salinity that mayarise using conventional serially-configured onshore RO.

Disposal of the brine steam may as discussed above be carried out usingan airlift and a brine dispersal conduit with openings, so as to removebrine from the disclosed system and disperse it into seawater into twoand more preferably into three dimensions. In preferred embodiments thebrine is dispersed into one or more substantial vertical portions of awater column or columns above or remote from the disclosed SROapparatus. Doing so can avoid the localized discharge of concentratedbrine dispersed by high-pressure point-source diffusers as commonly usedto disperse RO brine today, and the possible harm to marine life fromhigh salinity or diffuser shear forces. The brine removal conduit mayfor example be an upwardly-extending conduit supplied with air at aheight at or above the RO membranes, and preferably at or above theheight at which the injection air pressure in the brine removal conduitand the operating pressure of the product delivery conduit areequalized. If desired, more than one air introduction (e.g., airinjection) point may be employed. The air introduction point(s)desirably are located above the RO membranes so as to discourage theaccidental introduction of air into the RO membranes. The airintroduction point(s) may if desired be located above, below oralongside the prefilter, or in any combination thereof, with higherintroduction points typically requiring less energy to operate theassociated brine airlift, and lower introduction points providing agreater conduit length along which dilution, oxygenation or dispersionmay take place. The brine removal conduit preferably rises vertically orupwardly away from the air introduction point(s) and terminates at alesser depth than the air introduction point(s) so as to facilitateairlift of brine within the brine removal conduit. The brine removalconduit preferably has a substantial length beyond the air introductionpoint, e.g., at least 5 meters, at least 10 meters, at least 20 meters,at least 30 meters, at least 50 meters, at least 100 meters, at least500 meters or at least 1,000 meters. If desired, the brine removalconduit may divide or subdivide into a plurality of preferablyupwardly-directed arms each of which may carry airlifted brine anddisperse it into the surrounding seawater.

The brine-dispersing portion of the brine removal conduit preferablycontains a plurality of perforations or other openings in the conduitsidewall (or even one-way or other valves if desired) that provide brineoutlets. The brine outlets may be located below and more preferably arelocated above the air introduction point(s). The outlets may dispersebrine at a variety of depths, for example at depths above, below or bothabove and below a thermocline or halocline. The disclosed brine outletsare desirably sized, positioned and oriented to allow the dispersion ofbrine into the surrounding seawater and well away from the brine removalconduit. The brine outlets preferably are arranged over a substantiallength along the conduit (and more preferably are arranged over asubstantial vertical portion of a water column) of at least 5 meters,and in some embodiments, at least 10 meters, at least 20 meters, atleast 30 meters, at least 50 meters, at least 100 meters, at least 500meters or at least 1,000 meters. A variety of brine outlet openingshapes may be employed, including circular holes, slots, polygons,tapered ducts and other shapes. Vanes or other deflectors may bepositioned within the brine removal conduit to add turbulence to or todirect the brine through brine outlets. Brine can also be expelled fromthe brine outlets due to the expansion of rising air within the brineremoval conduit. If desired, some of the disclosed openings may besized, oriented or positioned to allow diluting seawater to be drawninto the moving brine stream within the conduit, e.g., via the Venturieffect, and thereby serve as brine-diluting seawater inlets. Whetherused to expel brine from the conduit or to draw diluting seawater intothe conduit, the disclosed openings may extend along a substantialextent (for example, at least 1%, at least 2%, at least 3%, at least 4%,at least 5%, at least 10%, at least 20%, at least 30% or at least 40%)of the brine removal conduit length beyond the first air introductionpoint. The size, orientation, frequency and positioning of the disclosedopenings may if desired vary along the length of the conduit, and mayfor example represent larger openings at distances close to the firstair introduction point and smaller openings at distances further fromthe desalination apparatus, or vice versa. One or more portions alongthe length of the brine removal conduit after the first air introductionpoint may be free of openings, for example to allow for enhancedoxygenation of moving brine within such portion. The furthest (andpreferably uppermost) end(s) of the brine removal conduit may be open,partially closed, or closed. Preferably there are sufficient brineoutlet openings to disperse the oxygenated brine stream over a largerarea (viz., into a larger volume of seawater) than would be obtainedusing point-source diffusers. In addition, the diffusion flow throughthe brine outlets preferably is not highly pressurized and thus does notcreate shear forces that might harm marine life.

The extent to which the brine is diluted, oxygenated or dispersed may becontrolled or influenced by a number of factors, including the number,size, shape and axial orientation of the disclosed openings, thepressure and volume of introduced brine airlift air, the respectivevelocities of the disclosed brine and brine airlift flows, and thepresence of turbulence at or after the air introduction point(s). Thebrine may if desired be dispersed above, below or both above and below athermocline or halocline.

The disclosed combination of an airlift and a brine delivery conduitwith appropriate openings can permit removal, dilution, oxygenation anddispersal of brine produced by the SRO system over a substantial arearemote from the SRO system. This can for example permit dispersal ofbrine high above the SRO system, enabling the system to rest on theocean floor without creating an unsafe environmental condition due tobrine, which is denser than seawater, pooling on the seafloor. Suchpooled brine could harm benthic-dwelling marine life, for example bycausing hyper-saline conditions on the ocean floor. Use of an airliftpump to disperse the brine can also save energy compared to the use ofmarine outfall lines or pressurized brine diffusers commonly employedwith shore-based RO plants. In addition, the disclosed airlift brinediffusion system can diffuse brine into a much larger area (viz., volumeof nearby water) than is the case for typical marine outfall lines orpressurized brine diffusers. The disclosed oxygenation can also reducethe incidence of naturally-occurring or otherwise induced hypoxia ordead zones in nearby seawater.

In a further preferred embodiment, the volume or pressure of airlift airsupplied to the disclosed brine removal conduit can be designed, set oradjusted so that during or following startup (e.g., during SROoperation) the brine airlift air will provide positive control of thevolume of saline water flowing through the RO membranes. This can helpprevent polarization at the boundary layer near the membrane surface,and will also discourage membrane fouling or scaling. In addition, suchcontrol can facilitate adjustment of the salinity of the brine stream,allow modification (e.g., reduction) of the brine stream airlift demand,or allow for sizing or resizing of pretreatment conditions andcapacities. In an especially preferred embodiment, the volume orpressure of brine airlift air is designed, set or adjusted to optimizethe RO membrane product water recovery rate and membrane health.

Expressed in terms of the air:brine volumetric ratio (determined shortlyafter the point at which air is injected into the brine removal conduit,and before taking into account the possible entry into the conduit ofseawater dilution streams via the disclosed openings), air:brine ratiosof at least about 1:99, at least about 5:95, at least about 10:90, atleast about 15:85 or at least about 20:80 may be employed. The air:brineratio may under some conditions be as high as about 99:1, as high asabout 95:5, or as high as about 90:10 but under normal operatingconditions typically will be less, for example up to about 60:40 or upto about 50:50.

In comparison to conventional marine outfall lines or multiportdiffusers, the disclosed brine dispersal system can provide improvedbrine dispersal with reduced capital and energy requirements. If thedisclosed brine airlift system is also used to set or adjust the volumeof saline water flowing through the RO membranes, then SRO systemperformance can be controlled at much lower capital cost than wouldtypically be required by the variable frequency drives and seawaterpumps typically used to control the pressure and flow rate of seawaterthrough RO membranes in conventional onshore systems.

A principal benefit of the overall disclosed SRO system is itssignificantly reduced energy requirements. The mechanical pressurizationof process water, the largest source of energy use in conventional ROdesalination, can be eliminated. For the disclosed exemplary SRO systemoperating with Dow FILMTEC membrane elements at approximately 680 mdepth, and with airflow adjusted to an 80% air fraction to provide anannular flow regime in the discharge water conduit and eliminate theneed for a mechanical pump to aid product flow, the energy consumptionmay be estimated. As discussed above, the current average energyconsumption for a shore-based RO unit is approximately 13.5 kWh/3785liters (1,000 US gallons). The energy consumption and associatedgreenhouse gas production to produce desalinated seawater using thedisclosed SRO system may be significantly reduced. The associatedcapital expenditures and operating expenditures can also besignificantly reduced, especially in comparison with those required foronshore RO desalination.

These and other advantages of the disclosed SRO system thus may includeone or more of:

-   -   Greatly reduced power consumption.    -   Reduced greenhouse gas emissions to desalinate a given quantity        of water.    -   Low temperature permeate and surplus airlift air each provide an        additional thermal value stream that may be used for cooling,        heat exchange, limiting or ameliorating the effects of        atmospheric warming, or further reducing greenhouse gas        emissions.    -   Elimination of both onshore and offshore high pressure water        pumps.    -   Elimination of the artificial high-pressure environment used in        conventional RO and the accompanying pressure vessels, high        pressure piping, and fittings.    -   Reduced operation and maintenance requirements through        elimination of parts, and especially the reduction of        highly-pressurized connections.    -   Greatly reduced number of parts requiring expensive alloys and        other exotic materials resistant to seawater corrosion.    -   Reduced or eliminated pretreatment equipment and its associated        operating capital and labor.    -   Reduced damage to RO membranes from mechanical pump vibrations.    -   Reduced localized brine emission.    -   Parallel rather than series membrane configurations with even        lower-salinity brine discharge.    -   Increased oxygenation of nearby seawater and reduction in        hypoxia.    -   Pipelines to shore that are over 50% smaller, as only product        water is sent onshore.    -   Reduced boron content in desalinated water, making it suitable        for agriculture without further treatment.    -   Reduced bacterial content and bacterial fouling due to the use        of deep-sea intake water that is relatively free of undesirable        organic or inorganic contaminants.    -   Reduced susceptibility to desalination disruption caused by        algal blooms.    -   Virtual invisibility from shore.    -   Reduced susceptibility to destruction due to adverse weather        events, fires, terrorism or volcanic eruptions.    -   Reductions by as much as 90% in required onshore real estate.    -   Suitability for deployment as an “Ocean Well” that can provide a        sustained freshwater supply without aquifer depletion.

Having thus described preferred embodiments of the present invention,those of skill in the art will readily appreciate that the teachingsfound herein may be applied to yet other embodiments within the scope ofthe claims hereto attached. The complete disclosure of all patents,patent documents, and publications are incorporated herein by referenceas if individually incorporated.

1. A desalination and cooling system comprising a submerged offshorereverse osmosis desalination apparatus having: a) one or more submergedosmotic membranes each having an inlet surface supplied with seawater atleast partially under hydrostatic pressure and an outlet surface thatprovides desalinated product water containing less than 0.5 parts perthousand (ppt) dissolved inorganic salts by weight to a submergedproduct water collector in fluid communication with the outletsurface(s), b) an at least partially submerged water discharge conduitin fluid communication with the collector and providing a pipeline thatdelivers the desalinated product water to shore; and an onshore cooleror heat exchanger in fluid communication with the water dischargeconduit, wherein the desalinated product water and onshore cooler orheat exchanger provide or improve the cooling of an onshore RankineCycle heat engine.
 2. A system according to claim 1 wherein thesubmerged apparatus relies wholly upon hydrostatic pressure to driveseawater through the osmotic membranes.
 3. A system according to claim 1wherein the submerged apparatus does not include submerged frictionalsliding surfaces, a submerged mechanical pump or a submerged valve.
 4. Asystem according to claim 1 wherein the submerged apparatus includes anair supply for removing water from the collector via airlift, and anairflow valve that controls airflow into the collector or into theconduit so that the desalinated product water is lifted by supplied airin an annular flow regime over 10% or more of the airlift depth.
 5. Asystem according to claim 4 wherein the supplied air lifts desalinatedproduct water from the collector through the discharge water conduitusing an air fraction of at least about 60%.
 6. A system according toclaim 4 wherein the supplied air lifts desalinated product water fromthe collector through the discharge water conduit using an air fractionof at least about 80%.
 7. A system according to claim 4 wherein thesupplied air lifts desalinated product water from the collector throughthe discharge water conduit at an air:water ratio sufficient to providean annular flow regime over at least the upper 20% of the dischargewater conduit.
 8. A system according to claim 4 wherein when carryingout continuous desalination and continuous airlift, the backpressuredownstream from the osmotic membranes, at the water discharge conduit,is less than about 290 psi (20 bar).
 9. A system according to claim 1wherein the osmotic membranes are not encased in a surroundingpressure-resistant housing.
 10. A system according to claim 1 whereinthe system supplies desalinated product water from the heat exchanger toa potable water storage system.
 11. A method for desalination andcooling, the method comprising operating a submerged reverse osmosisdesalination apparatus having: a) one or more submerged osmoticmembranes each having an inlet surface supplied with saltwater at leastpartially under hydrostatic pressure and an outlet surface that providesdesalinated product water containing less than 0.5 parts per thousand(ppt) dissolved inorganic salts by weight to a submerged product watercollector in fluid communication with the outlet surface(s), b) an atleast partially submerged water discharge conduit in fluid communicationwith the collector and providing a pipeline that delivers thedesalinated product water to shore; and using the desalinated productwater in an onshore cooler or heat exchanger in fluid communication withthe water discharge conduit to provide or improve the cooling of anonshore Rankine Cycle heat engine.
 12. A method according to claim 11wherein the submerged apparatus relies wholly upon hydrostatic pressureto drive seawater through the osmotic membranes.
 13. A method accordingto claim 11 wherein the submerged apparatus includes an air supply forremoving water from the collector via airlift, and an airflow valve thatcontrols airflow into the collector or into the conduit so that thedesalinated product water is lifted by supplied air in an annular flowregime over 10% or more of the airlift depth.
 14. A method according toclaim 13 wherein the supplied air lifts desalinated product water fromthe collector through the discharge water conduit using an air fractionof at least about 60%.
 15. A method according to claim 13 wherein thesupplied air lifts desalinated product water from the collector throughthe discharge water conduit at an air:water ratio sufficient to providean annular flow regime over at least the upper 20% of the dischargewater conduit.
 16. A method according to claim 13 wherein when carryingout continuous desalination and continuous airlift, the backpressuredownstream from the osmotic membranes, at the water discharge conduit,is less than about 290 psi (20 bar).
 17. A method according to claim 11wherein the osmotic membranes are not encased in a surroundingpressure-resistant housing.
 18. A method according to claim 11 furthercomprising supplying desalinated product water from the heat exchangerto a potable water storage system.